|molar fraction, amount fraction, amount-of-substance fraction
In chemistry, the mole fraction or molar fraction, also called mole proportion or molar proportion, is a quantity defined as the ratio between the amount of a constituent substance, ni (expressed in unit of moles, symbol mol), and the total amount of all constituents in a mixture, ntot (also expressed in moles):
It is denoted xi (lowercase Roman letter x), sometimes χi (lowercase Greek letter chi). (For mixtures of gases, the letter y is recommended.)
It is a dimensionless quantity with dimension of and dimensionless unit of moles per mole (mol/mol or mol ⋅ mol-1) or simply 1; metric prefixes may also be used (e.g., nmol/mol for 10-9). When expressed in percent, it is known as the mole percent or molar percentage (unit symbol %, sometimes "mol%", equivalent to cmol/mol for 10-2). The mole fraction is called amount fraction by the International Union of Pure and Applied Chemistry (IUPAC) and amount-of-substance fraction by the U.S. National Institute of Standards and Technology (NIST). This nomenclature is part of the International System of Quantities (ISQ), as standardized in ISO 80000-9, which deprecates "mole fraction" based on the unacceptability of mixing information with units when expressing the values of quantities.
The sum of all the mole fractions in a mixture is equal to 1:
Mole fraction is numerically identical to the number fraction, which is defined as the number of particles (molecules) of a constituent Ni divided by the total number of all molecules Ntot. Whereas mole fraction is a ratio of amounts to amounts (in units of moles per moles), molar concentration is a quotient of amount to volume (in units of moles per litre). Other ways of expressing the composition of a mixture as a dimensionless quantity are mass fraction and volume fraction are others.
Mole fraction is used very frequently in the construction of phase diagrams. It has a number of advantages:
Differential quotients can be formed at constant ratios like those above:
The ratios X, Y, and Z of mole fractions can be written for ternary and multicomponent systems:
These can be used for solving PDEs like:
This equality can be rearranged to have differential quotient of mole amounts or fractions on one side.
Mole amounts can be eliminated by forming ratios:
Thus the ratio of chemical potentials becomes:
Similarly the ratio for the multicomponents system becomes
The mass fraction wi can be calculated using the formula
where Mi is the molar mass of the component i and M̄ is the average molar mass of the mixture.
The mixing of two pure components can be expressed introducing the amount or molar mixing ratio of them . Then the mole fractions of the components will be:
The amount ratio equals the ratio of mole fractions of components:
due to division of both numerator and denominator by the sum of molar amounts of components. This property has consequences for representations of phase diagrams using, for instance, ternary plots.
Mixing binary mixtures with a common component gives a ternary mixture with certain mixing ratios between the three components. These mixing ratios from the ternary and the corresponding mole fractions of the ternary mixture x1(123), x2(123), x3(123) can be expressed as a function of several mixing ratios involved, the mixing ratios between the components of the binary mixtures and the mixing ratio of the binary mixtures to form the ternary one.
Multiplying mole fraction by 100 gives the mole percentage, also referred as amount/amount percent [abbreviated as (n/n)% or mol %].
The conversion to and from mass concentration ρi is given by:
where M̄ is the average molar mass of the mixture.
The conversion to molar concentration ci is given by:
where M̄ is the average molar mass of the solution, c is the total molar concentration and ρ is the density of the solution.
The mole fraction can be calculated from the masses mi and molar masses Mi of the components:
In a spatially non-uniform mixture, the mole fraction gradient triggers the phenomenon of diffusion.